In particular applications, there is a need for an insulator material, which is of a light weight, in particular when exposed to a high acceleration, for example in computer tomography devices, in which the high voltage parts are rotating in a high speed, which results in a high radial acceleration of the components. Therefore, there is a need for a light weight material in order to reduce the moved masses in order to reduce the forces due to a high radial acceleration. From EP 1 176 856, it is known that for a solid high voltage insulation material, e.g. based on epoxy resin, which shall have a low weight, hollow micro-spheres are used as a sort of filler. For an optimal high voltage construction it is necessary to balance the design parameters of these hollow micro-spheres. To get the lowest weight by a given material of the micro-spheres, e.g. glass, it would be useful to realise relative large hollow micro-spheres with a thin wall thickness to get the lowest overall weight when these micro-spheres were put into the epoxy resin as a filler together with the hardener and other ingredients like coupling agents, etc.
However, the diameter of the micro-spheres influence the dielectric strength in such a way that the larger the diameter is, the lower is the electric strength owing to partial discharges (PDs), which occur in gaseous enclosures inside of a solid material due to an increased electrical field within the gaseous spaces in form of gas filled hollow micro-spheres. These partial discharges start from a certain ignition voltage onwards, which depends on the gas pressure at the acceleration gap within the hollow micro-sphere to start an ionisation process which leads to an electron avalanche hitting the inner surface of the micro-sphere. This process is well known from the theory as partial discharge process. From a certain energy over a certain time onwards, this electrical erosion process caused by partial discharges destroys first the wall of the, e.g. glass of the hollow micro-spheres, depending on the wall thickness and next the surrounding epoxy resin matrix resulting in a total breakdown of the insulation material. These effects are also known from other solid insulation materials, for example, for high voltage power cables having a polymer insulation material.
To prevent these partial discharges, the diameter and by this, the acceleration gap, within the hollow micro-spheres has to be reduced to such an amount that no partial discharges may occur. Since the hollow micro-spheres are nominally filled with a gas like, for example, air, N2, CO2, SO2, which depends on the production process, the so-called Paschen law is valid for calculating the ignition voltage for the partial discharge. The ignition voltage is for small acceleration gaps and low pressure inverse proportional to the gas pressure p multiplied by the acceleration gap distance d, wherein the acceleration gap corresponds to the diameter of the hollow spheres.
That means that either the pressure or the diameter has to be made to zero to get the highest ignition voltage for preventing the partial discharge. The ignition voltage has to be higher than the nominal voltage which is put from the overall construction divided by the inner voltage dividers to the specific micro-spheres, which corresponds to the theory of partial discharge breakdown.
Reducing the diameter means that the relation of the wall thickness to the gas filled volume becomes worse and by that the weight of the total hybrid material comes up.